Properties (getters and setters) in Ceylon

Question

How do I define a property in Ceylon? I know I can write getName and setName functions to get and set a backing variable:

class Circle(shared variable Float radius) {
    shared Float getArea() {
        return pi * radius ^ 2;
    }

    shared void setArea(Float area) {
        radius = sqrt(area / pi);
    }
}

value circle = Circle(4.0);
circle.setArea(10.0);
print(circle.getArea());

But I would like to be able to provide attribute-like access to the property:

value circle = Circle(4.0);
circle.area = 10.0;
print(circle.area);

How do I do this in Ceylon?

Answer 1

Getters are declared like defining a function with no parameter list. The getter body then behaves like a normal function and must return the calculated value of the property.

variable String local_var = "Hello world!";

// Getter
String name1 {
    return local_var;
}

// Also getter, using functional syntax for body
String name2 => local_var;

Setters are declared using the assign keyword. The setter body then behaves like a void function, performing whatever mutation of the local environment is appropriate and must not return a value. The incoming value being assigned can be referred to in the body via the name of the property.

// Setter
assign name1 {
    local_var = name1; // name1 here is value being assigned
}

// Also setter, using modified functional syntax
assign name2 => local_var = name2;

Unlike most programming languages, properties can be top level members of the package, not just members of a class:

class Circle(shared variable Float radius) {
    // Class-level getter
    shared Float area {
        return pi * radius ^ 2;
    }

    // Class-level setter
    assign area {
        radius = sqrt(area / pi);
    }
}

Circle window = Circle(1.0);

// Top-level getter
Circle outer_window => Circle(window.radius * 2);

// Top-level setter
assign outer_window => window.radius = outer_window.radius / 2;

Answer 2

Additional comment: on the Java backend, Ceylon getters and setters compile to Java getters and setters (using the standard getFoo/setFoo names, or isFoo for Boolean properties). Defining regular functions named like getters and setters, like getArea and setArea in the question, is strongly discouraged, and if you do define them, their backing functions will actually be called something different (e. g. $getArea) to avoid collision with the getters and setters generated automatically by the compiler.